Endoluminal endovascular grafting has developed as an alternative to conventional vascular surgery. It involves the placement of a prosthesis within a body lumen, such as a blood vessel or other body duct. This obviates the need for surgically incising, removing, replacing, or bypassing a defective vessel.
With endoluminal grafting, the first step is to access the defective vessel either percutaneously or using surgical cut-down techniques. (Percutaneous entry is through the skin, and surgical cut-down entry is directly into an artery itself, which has been surgically exposed.
After accessing the vessel, a tubular prosthesis is delivered, by catheter, to a particular location within the vessel. Typically, a prosthesis is either a stent with a fabric graft covering or a stent alone. The prosthesis is deployed at the desired location where it expands to a predetermined size and presses outwardly against the lumen walls of the vessel.
A particular use of endoluminal endovascular grafting is to hold arteries open after angioplasty or atherectomy. Arteries treated by such procedures are subject to restenosis, dissections, and intimal flap formation, which can impede blood flow. An artery treated by angioplasty or atherectomy may also show elastic recoil and later restenosis. Another use is to create a new wall inside an aneurysmatic artery.
The use of vascular stents alone (without a graft covering) has been shown to control elastic recoil and formation of intimal flaps. Two main types of stents are currently available: a balloon-expandable type, made from a malleable metal, and a self-expanding type made from an elastic metal.
The balloon-expandable types are usually made from malleable stainless steel or tantalum. Such a stent is disclosed in U.S. Pat. No. 5,133,732, issued to Wiktor. The malleability of the metal allows the stent to be compressed around an angioplasty balloon, and expanded at the correct location in the vasculature by inflating the balloon.
The advantage of balloon-expandable stents is the ability to be expanded to an exact size simply by choosing a correct balloon size. Such stents can be tailored to fit the diameter of the lumen of the vessel.
A disadvantage of such stents, however, is that they are non-elastic and non-compliant. If they are crushed or kinked by an external force, they do not recover their shape. If they are physically crushed, they remain crushed. Further angioplasty is required in order to re-establish blood flow.
In addition, if they are overdialated during angioplasty, they remain overdialated. If such stents are covered with fabric grafts of Dacron.backslash. or polytetrafluoroethylene (PTFE), the fabric that is overexpanded remains overexpanded. This creates creases that can push the stent into the lumen or protrude through the stent frame into the lumen. Furthermore, such stents shorten in length when they are released by as much as 30%. Angioplasty cannot remedy these problems.
Elastic stents made from stainless steel are self-expanding and remain elastic and compliant. For stents made from normal spring metals, however, it is difficult to control the outward force on the vessel wall. According to Hook's law of elasticity, the outward force exerted by elastic stents made of spring metals increases with the amount of stent compression.
To address this problem, it is known to use nitinol as the material from which a stent is made. Nitinol removes the problem of variable outward force because it has the property, at body temperature, of pseudoelasticity, or superelasticity. In the superelastic state, and within certain limits, the outward force exerted by a nitinol stent remains constant, regardless of the strain in the stent.
A disadvantage of stents, however, is that tissue can grow from the lumen walls through the spaces defined by the wire forming the stents. In order to prevent such tissue ingrowth, fabric grafts have been developed to cover a stent. Materials used or advocated for this purpose include: woven or knitted polyester, PTFE, polyurethane, and other elastomeric polymers.
Some fabric covering materials, however, may be thrombogenic and thicker than desired. A thick covering is not suitable for small calibre arteries, such as femeral, renal, popliteal, carotid, and coronary arteries. Such arteries require extremely small diameter prostheses. In addition, small calibre arteries often have geometric configurations making it difficult to suitably open the artery with a prosthesis. For example, the artery may taper, preventing a prosthesis from being inserted.
U.S. Pat. No. 5,123,917, issued to Lee, describes a method of trapping a scaffold member between two separate tubes which are bonded together. This multi-part construction adds thickness to the graft and has the possible disadvantage of breakdown of the bonding adhesive with the resultant separation of the various parts of the structure.
Other patents, such as U.S. Pat. No. 5,282,824, issued to Gianturco, and U.S. Pat. No. 4,739,762, issued to Palmaz, describe stents that are embedded in a sleeve of plastic covering the outer surface of the stent. Such devices have the disadvantage discussed above of exposure of the inner surface of the stent to the blood in the vessel, thus presenting an uneven surface contributing to turbulent flow.
None of the known devices is a satisfactory prosthesis for use in small calibre arteries. A prosthesis that is small enough and flexible enough for use in such arteries is desirable.